1. Field of the Invention
This invention relates to an apparatus for use in accurately determining a wellbore caliper. In particular the invention relates to the determination of wellbore caliper while a drilling process is taking place. In a practical embodiment, this is achieved by using a plurality of orthogonally mounted accelerometers.
2. Description of the Related Art
Typically, a wellbore extending through a formation is not straight, but rather extends in a snake-like fashion through the formation. Such wellbores are often of spiraling form resulting from the rotary motion of the drill bit. However, the wellbore may also take other forms, for example, as a result of the drill bit being deflected from its original path as a result of encountering a change in the structure of the formation through which the wellbore is being drilled. Even wellbores which are regarded as being straight often have variations in deviation and direction. Although these variations may be small, they can still be of significance when completing a wellbore. By way of example, it is usual to line a wellbore of 8xc2xdxe2x80x3 diameter using a casing having an outer diameter of 7xe2x80x3. Clearly, if the wellbore is exactly straight, this gives a radial clearance of only xc2xexe2x80x3. Obviously, where the wellbore is not exactly straight, then there may (be regions where there is less clearance, or regions where the provisions of such a casing is not practical and other lining techniques may need to be used.
In practice, wellbores are very rarely exactly straight, indeed with the advent of steerable drilling systems highly deviated and horizontal wellbores are widely used in order to enhance reservoir production. The positioning of a completion string such as a wellbore casing within such a wellbore can be a very difficult operation and may result in damage to the completion string. Even where the completion string is not damaged, there is an increased likelihood of impaired production rates.
It will be appreciated from the description above that the geometry and orientation of the wellbore, as well as the way a completion string will sit in the wellbore, play a very important part in determining the effectiveness of the completion during clean up, treatment, cementing/isolation, and production.
A number of techniques are known to permit the measurement of wellbore shape. One such technique involves the use of a tool known as a dipmeter which includes sensors arranged to measure variations in the conductivity of the formation. The dipmeter has calipers arranged to measure the size of the wellbore as the dipmeter passes along the length of the wellbore. Other sensors arranged to measure the deviation and direction of the wellbore may also be provided. In use, the dipmeter is passed along the length of the wellbore and readings are taken using the various sensors. The readings are logged along with the position of the dipmeter at the time the readings are taken and this information is subsequently used to produce a three-dimensional image of the wellbore.
Other tools are also known for use in measuring the shape of the wellbore. For example, a tool known as a borehole geometry tool can be used. A tool of this type is similar to a dipmeter but does not include sensors for measuring formation conductivity. Another tool is an ultrasonic borehole imaging (UBI) tool. This tool is used in conjunction with a general purpose inclinometry tool to generate data representative of the wellbore shape and size which data can, if desired, be used to produce a three-dimensional image of the wellbore.
It will be appreciated that knowledge of what is likely to happen downhole as a completion string is inserted into a wellbore is useful in deciding how to complete a wellbore.
Accurate measurement of the wellbore caliper using the above-described devices can only be achieved after drilling. Measurement while drilling is not practical as it is not possible to determine the absolute position of the tool being used to generate the desired data. Further, where a UBI tool is used, the tool must be rotated relatively slowly as the sensitivity of the tool decreases with increasing speed, making the tool unsuitable for use in a measurement while drilling system.
Measurement of a number of drilling parameters while drilling can be achieved. For example, WO99/36801 describes an arrangement for nuclear magnetic resonance (NMR) imaging of a wellbore. Such imaging is useful as it can be used to derive information representative of the porosity, fluid composition, the quantity of moveable fluid and the permeability of the formation being drilled. In order to produce useful data, it is important that the sensor of the arrangement is either stationary or is only moving relatively slowly. Where fast movement is occurring, the results are less useful in determining the values of the parameters as there is an increased risk of significant errors in the results. In order to determine whether or not the NMR readings taken using the tool can be used, the tool is provided with sensors for use in monitoring the motion of the tool. One example of a suitable sensor arrangement is to provide the tool with accelerometers and a suitable control arrangement. The accelerometer readings can be used to produce data representative of the motion of the tool, and the control arrangement can be used to inhibit the production of NMR data when the motion of the tool is such that the NMR readings would be likely to include significant errors. Alternatively, the control arrangement may be arranged to allow the NMR readings to be made to flag the readings that are likely to contain errors.
According to the present invention there is provided an accelerometer caliper while drilling arrangement comprising a drill bit having an axis of rotation and a gauge region, a caliper tool body, a first accelerometer mounted upon the caliper tool body and arranged to measure acceleration in a first direction, and a second accelerometer mounted upon the caliper tool body and arranged to measure acceleration in a second direction orthogonal to the first direction, wherein the caliper tool body and the drill bit are coupled to one another in such a manner that the first and second accelerometers are mounted in a known relationship to the drill bit.
As the accelerometers are mounted in a known relationship to the drill bit, and as the drill bit defines the edges of the bore, the positions of the accelerometers are known and the acceleration readings taken using the accelerometer can be used to ascertain the shape of the wellbore.
Although as described above, only two orthogonally mounted accelerometers are required, it will be appreciated that if a greater number of accelerometers are provided, then it may be possible to increase the accuracy with which caliper readings can be taken. In a preferred arrangement, three accelerometers are used, but it will be appreciated that the invention is not restricted to arrangements including three accelerometers.
It is thought that the accelerometer caliper while drilling tool will be able to take wellbore caliper diameter measurements with an accuracy of up to about +/xe2x88x920.06xe2x80x3.
If desired, the caliper tool body may form part of the drill bit.